A simulated photomask. The thicker features are the integrated
circuit that is desired to be printed on the wafer. The thinner
features are assists that do not print themselves, but help the
integrated circuit print better out-of-focus. The zig-zag appearance
of the photomask is because optical proximity correction was applied
to it to create a better print.

Lithographic photomasks are typically transparent fused silica blanks
covered with a pattern defined with a chrome metal-absorbing film.
Photomasks are used at wavelengths of 365 nm , 248 nm, and 193 nm.
Photomasks have also been developed for other forms of radiation such
as 157 nm, 13.5 nm (EUV ),
X-ray , electrons , and ions ; but these
require entirely new materials for the substrate and the pattern film.

A set of photomasks , each defining a pattern layer in integrated
circuit fabrication , is fed into a photolithography stepper or
scanner , and individually selected for exposure. In double patterning
techniques, a photomask would correspond to a subset of the layer
pattern.

In photolithography for the mass production of integrated circuit
devices, the more correct term is usually PHOTORETICLE or simply
RETICLE. In the case of a photomask, there is a one-to-one
correspondence between the mask pattern and the wafer pattern. This
was the standard for the 1:1 mask aligners that were succeeded by
steppers and scanners with reduction optics. As used in steppers and
scanners, the reticle commonly contains only one layer of the chip.
(However, some photolithography fabrications utilize reticles with
more than one layer patterned onto the same mask). The pattern is
projected and shrunk by four or five times onto the wafer surface. To
achieve complete wafer coverage, the wafer is repeatedly "stepped"
from position to position under the optical column until full exposure
is achieved.

Features 150 nm or below in size generally require phase-shifting to
enhance the image quality to acceptable values. This can be achieved
in many ways. The two most common methods are to use an attenuated
phase-shifting background film on the mask to increase the contrast of
small intensity peaks, or to etch the exposed quartz so that the edge
between the etched and unetched areas can be used to image nearly zero
intensity. In the second case, unwanted edges would need to be trimmed
out with another exposure. The former method is attenuated
phase-shifting, and is often considered a weak enhancement, requiring
special illumination for the most enhancement, while the latter method
is known as alternating-aperture phase-shifting, and is the most
popular strong enhancement technique.

As leading-edge semiconductor features shrink, photomask features
that are 4× larger must inevitably shrink as well. This could pose
challenges since the absorber film will need to become thinner, and
hence less opaque. A recent study by IMEC has found that thinner
absorbers degrade image contrast and therefore contribute to line-edge
roughness, using state-of-the-art photolithography tools. One
possibility is to eliminate absorbers altogether and use "chromeless"
masks, relying solely on phase-shifting for imaging.

The emergence of immersion lithography has a strong impact on
photomask requirements. The commonly used attenuated phase-shifting
mask is more sensitive to the higher incidence angles applied in
"hyper-NA" lithography, due to the longer optical path through the
patterned film.

MASK ERROR ENHANCEMENT FACTOR (MEEF)

Leading-edge photomasks (pre-corrected) images of the final chip
patterns magnified by 4 times. This magnification factor has been a
key benefit in reducing pattern sensitivity to imaging errors.
However, as features continue to shrink, two trends come into play:
the first is that the mask error factor begins to exceed one, i.e.,
the dimension error on the wafer may be more than 1/4 the dimension
error on the mask, and the second is that the mask feature is
becoming smaller, and the dimension tolerance is approaching a few
nanometers. For example, a 25 nm wafer pattern should correspond to a
100 nm mask pattern, but the wafer tolerance could be 1.25 nm (5%
spec), which translates into 5 nm on the photomask. The variation of
electron beam scattering in directly writing the photomask pattern can
easily well exceed this.

PELLICLES

The term "pellicle" is used to mean "film," "thin film," or
"membrane." Beginning in the 1960s, thin film stretched on a metal
frame, also known as a "pellicle," was used as a beam splitter for
optical instruments. It has been used in a number of instruments to
split a beam of light without causing an optical path shift due to its
small film thickness. In 1978, Shea et al. at
IBM patented a process
to use the "pellicle" as a dust cover to protect a photomask or
reticle(hence will all be called "photomask" in the rest of this
chapter) In the context of this entry, "pellicle" means "thin film
dust cover to protect a photomask".

Particle contamination can be a significant problem in semiconductor
manufacturing. A photomask is protected from particles by a pellicle
– a thin transparent film stretched over a frame that is glued over
one side of the photomask. The pellicle is far enough away from the
mask patterns so that moderate-to-small sized particles that land on
the pellicle will be too far out of focus to print. Although they are
designed to keep particles away, pellicles become a part of the
imaging system and their optical properties need to be taken into
account. Pellicles material are Nitrocellulose and made for various
Transmission Wavelengths. Pellicle Mounting Machine MLI

LEADING COMMERCIAL PHOTOMASK MANUFACTURERS

The
SPIE Annual Conference,
Photomask Technology reports the SEMATECH
Mask Industry Assessment which includes current industry analysis and
the results of their annual photomask manufacturers survey. The
following companies are listed in order of their global market share
(2009 info):

Worldwide photomask market was estimated as $3.2 billion in 2012 and
$3.1 billion in 2013. Almost half of market was from captive mask
shops (in-house mask shops of major chipmakers).

The costs of creating new mask shop for 180 nm processes were
estimated in 2005 as $40 million, and for 130 nm - more than $100
million.

The purchase price of a photomask, in 2006, could range from $1,000
to $100,000 for a single high-end phase-shift mask. As many as 30
masks (of varying price) may be required to form a complete mask set .

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